That only protects the digital signal inputs, not the analogue signals. Note the asterisk that is on each of the digital control pins.Yes I thought I needed extra protection, but then I checked 4067 analog multiplexer data sheet. https://www.ti.com/lit/ds/symlink/cd4067b.pdf
Every input is protected internally (see below) and since I am using Vdd=3.3V supply for the analog multiplexer, I seem to have that covered, especially considering the fact that I am supplying SS49E with something like 4.5V only (note bipolar transistor keys in the schematics) and at full swing SS49E outputs not more than 3.8V, so I am within 4067 specs.
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You want to protect the Teensy from over voltage. It didn't want more than 3.3V on its ADC pins. The AH49E can send +5V and the CD4067 can pass it through without the clamping that you had previously assumed so you can get 5V at the Teensy. You can test this by disconnecting from the Teensy then applying a reverse magnetic field to a sensor then measuring the voltage at the output of the CD4067.As I wrote: spec say max voltage on any pin Vdd + 0.5V. I am within specs even under most extreme situation (wrong assembly, reverse orientation of magnets, keys with wrong orientation pressed to max).
Under normal operation I am always below 2.5V.
I guess that if I ever made this "commercially" I would put those extra diodes just for the "peace of mind", but with my prototype build, I am not worried.